Over the last quarter-century there has been a drastic increase in the level of integration of integrated circuits (ICs). At the same time, there has been a corresponding significant decrease in the feature size of ICs. For example, the width of a MOSFET (metal-oxide-semiconductor field-effect transistor) gate is presently on the order of 45 nm and is projected to be 18 nm in 2010. This is less than 1/500 the width of a human hair. IC components have not only dramatically reduced in size, but have also reduced in power consumption. ICs are typically made using CMOS (complementary metal-oxide semiconductor) circuitry, which is made of dual n-FET and p-FET devices. CMOS circuitry consumes much less power than either purely nMOS or purely pMOS circuitry.
Reduction in both size and power consumption of ICs has led to the recent proliferation of wireless IC technology, which was not available just a decade ago. Today, there is a diversity of devices using low-power wireless circuits, including laptop computers, cell phones, MP3 players, smart phones, telephony headsets, headphones, routers, gaming controllers, mobile Internet adaptors, and spy cameras, to name just a few. Of course, each of these devices requires some sort of standalone power supply to work. Typically power supplies for these devices are electrical batteries, often replaceable batteries.
A wireless technology field of significant current interest, and that is the target for much research, is the field of wireless sensor networks. Indeed, researchers envision the future to include a widespread adoption of wireless sensor networks (WSNs). In WSNs, wireless sensors will be distributed throughout a particular environment to form an ad-hoc network or mesh that relays measurement data to a central hub. The particular environment could be any one of an automobile, an aircraft, a factory, and a building, among many others. A WSN will comprise several to tens of thousands wireless sensor nodes that will operate using multi-hop transmissions over short distances. Each wireless node will generally include a sensor, wireless electronics and a power source. The result will be the creation of an intelligent environment responding to its conditions and inhabitants, if any.
A wireless sensor node, like the other wireless devices mentioned above, needs some sort of standalone electrical power supply to provide power to the electronics aboard that node. Conventional batteries, such as lithium-ion batteries, zinc-air batteries, lithium batteries, alkaline batteries, nickel-metal-hydride batteries and nickel-cadmium batteries, could be used. However, for wireless sensor nodes designed to function beyond the typical lifetime of such batteries, at some point the batteries would have to be replaced. This could cause significant problems and expense depending on the number of nodes at issue and the accessibility of those nodes, not to mention the need to dispose of the batteries. Consequently, alternatives to batteries and other types of power supplies needing periodic attention, such as micro-size fuel cells, will be desirable for many WSNs.
Such alternative standalone power supplies would typically rely on scavenging (or “harvesting”) of energy from the ambient environment of a wireless sensor node. For example, if the wireless sensor node is exposed to sufficient light, the alternative standalone power supply could include photoelectric or solar cells. Alternatively, if the wireless sensor node is exposed to sufficient air movement, the alternative power supply could include a micro-turbine for harvesting power from the moving air. Other alternative standalone power supplies could also be based on temperature fluctuations, pressure fluctuations or other environmental influences.
However, there will be many instances when the ambient environment does not include sufficient amounts of light, air movement, temperature fluctuation and pressure variation to provide enough power to power a particular wireless sensor node. However, the sensor node may be subjected to fairly predictable and/or constant vibrations, for example, emanating from the structure supporting the node or to which the node is attached. In this case, a vibrational energy scavenger (or harvester) that essentially converts vibrational energy into electrical energy can be used.
A particular type of vibrational energy harvester utilizes resonant beams that incorporate a piezoelectric material that generates electrical charge when strained during resonance of the beams caused by ambient vibrations (driving forces). One shortcoming of many conventional piezoelectric vibrational energy harvesters (PVEHs) is that they are minimally dampened devices having high quality factors (Q). Thus, they are effective over only very small bandwidths of vibrational frequency. This becomes problematic under any one or more of a variety of circumstances, such as when the wireless sensor node is subjected to temperature variations that change the tuning of the PVEH, when the frequency of the ambient vibrations varies over time and when the manufacturing methods used to make the PVEH cause variation in the as-built tuning of the PVEH.